The present invention relates in general to vehicles with electric propulsion, and, more specifically, to monitoring of an output voltage from a battery pack by a microcontroller that is digitally isolated from the high voltage battery circuits.
The DC power source (e.g., a battery) and other elements of electric drives for hybrid or electric vehicles require monitoring in order to maximize efficiency and performance as well as to detect potential malfunctions. Common battery types such as lithium ion (Li-Ion) use a large number of cells stacked together into a battery pack. Besides monitoring the total voltage output by a battery pack, each cell is typically monitored individually to determine their voltage production and other parameters. The temperature of each cell is also monitored in order to protect against overheating.
It is very challenging to reliably monitor the various battery conditions because of the high-voltage levels involved, the range of intermediate voltages at which respective cells operate within the stack, and the high levels of accuracy required. Various battery monitoring integrated circuit devices have been developed commercially for use in the vehicle environment. Examples of a commercially available battery monitoring IC device include the AD7280A device available from Analog Devices, Inc., of Norwood, Mass., the LTC6804 devices available from Linear Technology Corporation of Milpitas, Calif., and the ISL94212 Multi-Cell Li-Ion Battery Manager available from Intersil Corporation of Milpitas, Calif. In addition to a plurality of inputs for directly monitoring respective battery cells, the known IC devices include several analog inputs for measuring the outputs of thermistor circuits used as temperature sensors for the respective battery cells.
Each commercially available battery monitoring IC is designed with a specific number of measurement channels from monitoring respective battery cells. The AD7280A mentioned above has 6 channels, and the LTC6804 and ISL94212 each has 12 channels. In order to achieve the desired output voltage levels, a typical Li-Ion battery for an electric vehicle often stacks as many as about 96 cells. In order to monitor individual cells, the battery monitoring ICs are likewise stacked between the positive and negative output busses of the battery pack. Since no single monitoring IC is connected to both busses, additional components are used in order to measure the full output voltage of the battery pack. Furthermore, additional components are used to measure the full output voltage of the battery pack rather than summing the individual cell voltages (or group of cells) as measured by the IC because of sampling time and microcontroller chronometric loading reasons (e.g., the large number of serial messages that would be required).
The measured parameters of the battery cells, battery pack, and associated devices are all used by a main microcontroller or microprocessor for performing battery management and communication. The main micro is typically located in a discrete battery control module or box that interfaces with other vehicle components such as a vehicle system/powertrain controller or a driver interface module. Consequently, the main micro uses a chassis ground for its voltage reference. The chassis ground is isolated from the main battery pack's reference which is provided at the negative battery bus.
The battery monitoring ICs deployed with the battery pack must monitor the battery cells while being referenced to the negative bus. Therefore, the battery monitoring ICs and any other monitoring devices connected in the high-voltage domain must communicate with the main micro through domain-crossing elements that provide digital isolation between the high-voltage battery domain and the chassis ground domain (i.e., low voltage domain) of the main micro. In order for the main micro to both control the monitoring elements and receive the resulting measured data, relatively expensive components such as photoMOS transistors and a dedicated, high-voltage-referenced analog-to-digital (A/D) converter integrated circuit are typically employed. It would be very desirable to avoid the use of the expensive add-on components while maintaining robust detection of the output voltage from the battery pack.